1. Field
The invention relates to a cryostat arrangement for an electrical power conditioner, more particularly, to a cryostat for use with superconducting transformers, superconducting fault current limiters, superconducting power devices for phase correction, etc.
2. Description of Related Art
Cryostats for electrical power conditioners are known which may be provided in one of the following two variants: (i) a cryostat comprising no opening for accommodating a ferromagnetic limb, and (ii) a cryostat with one or more openings for accommodating one or more ferromagnetic limbs.
A cryostat for electric power conditioner of type (i) is described for instance in EP 1 544 873 A2. The cryostat comprises external walls in contact with an ambient medium, internal walls in contact with a cooled medium, a thermal insulating gap formed between the external walls and the internal walls, the insulating gap comprising a thermal insulation. The thermal insulation is provided in this technical solution by vacuum; the insulating gap is evacuated.
The external walls comprise one cylindrical wall and two flat walls; a first external wall from the top (in the cap flange) and a second external wall from the bottom. In the same way, the internal walls comprise one cylindrical wall and two flat walls; a first internal wall from the top (in the cap flange) and a second internal wall from the bottom. The cryostat comprises also means for forming a liquid from a gas.
Both the external walls and the internal walls comprise a uniform structure and are made from a homogeneous metallic sheet.
A similar construction of a cryostat for electric power conditioners is disclosed in WO 94 003 955 A1. The cryostat comprises practically the same features as in the EP 1 544 873 A2 with a difference that the cap flange is converted in an upper external wall and an upper internal wall.
A cryostat with a central axial opening is also disclosed in U.S. Pat. No. 5,847,633 A which has similar features to those cryostats discussed above.
A cryostat for electric power conditioner of type (ii), i.e. with an internal opening for a ferromagnetic limb, is disclosed in U.S. Pat. No. 5,107,240 A, for example. The cryostat comprises external walls in contact with an ambient medium, internal walls in contact with a cooled medium and a thermally insulating gap formed between the external walls and the internal walls. The thermally insulating gap comprises a thermal insulation provided by vacuum.
The external walls comprise two cylindrical walls and two flat walls: a first external flat wall forms the top side (in the cap flange) and a second external flat wall forms the bottom side. The internal walls comprise two cylindrical walls and a flat wall forms the bottom side.
The external walls and the internal walls comprise a uniform structure and are formed from a homogeneous glass fiber reinforced vinyl polyester resin (FRP). As mentioned above, a vacuum is created between these FRP walls to provide the thermal insulation.
The ambient medium in this cryostat is provided by a ferromagnetic shell which serves for guiding of a magnetic flux. This material is kept practically at ambient temperature by means of natural or forced heat exchange. The ferromagnetic shell may play also a role of a fixture for the external walls. This fixture may provide an external mechanical stabilization of the cryostat (e.g. in case of electromagnetic forces) and may also allow, nevertheless, forces caused by presence of the vacuum between the external walls and the internal walls to be compensated.
In order to provide such compensation in the cryostat for electric power conditioner disclosed in U.S. Pat. No. 6,324,851 B1 the thermal insulating gap is filled, at least in part with a solid thermal insulator. The cryostat comprises external walls being in contact with an ambient medium, internal walls being in contact with a cooled medium, a thermal insulating gap formed between the external walls and the internal walls, the insulating gap comprising a thermal insulation.
In the arrangement disclosed in U.S. Pat. No. 6,324,851 B1, the thermal insulation is provided partly by the solid thermal insulation and partly by a vacuum.
The external walls comprise a plurality of side walls defining a plurality of openings, each of which can accommodate a ferromagnetic limb and two flat walls. The internal walls comprise also a plurality of side walls and two flat walls. Furthermore, the cryostat comprises means for filling in with a liquidized gas or/and means for gas liquidizing.
The external walls and the internal walls comprise a uniform structure. The external walls are made of metal sheet. The internal walls are made of a fiber composite material comprising properties of an electrical insulator.
The solid thermal insulator plays a role of a spacer and is load bearing. The solid thermal insulator is able to transmit the internal pressure acting on the internal walls to the external walls. The thermal conductivity of the solid thermal insulator (e.g. of 2 mW/(K×m)) is relatively low, but, however, not low enough to be compared to the vacuum insulation.
Comparing different technical solutions of the actual state of the art one may conclude that there is an obvious dilemma: (a) to employ a cryostat with the metallic walls which may provide an excellent and long-lifetime vacuum insulation and needs practically no maintenance, but causes high eddy currents and therefore leads to elevated cooling losses, or (b) to employ a cryostat with insulating walls (i.e. the walls without eddy current losses) which are much less vacuum tight and, as a result, the cryostat has to be periodically pumped in order to maintain a sufficient vacuum. Thus, in the latter case an additional periodic maintenance a special service means are needed while the lifetime of the cryostat is shorter.
Further improvements to the arrangements of cryostats for use in electrical power conditioners which overcome at least some of these disadvantages are desirable.
It is, therefore, desirable to provide an improved cryostat for use in electrical power conditioners avoids at least some of these disadvantages.